DE 102 45 983 A1 describes a ball joint with a housing, two bearing socket elements arranged in the housing, a joint body comprising a pin and a joint ball which fits between the two bearing socket elements, and a housing bottom positioned on the side of the pin facing away from the pin. Between a first of the two bearing socket elements and the housing bottom is arranged an adjustable clamping device by means of which the pre-stress with which the joint body is held between the bearing socket elements can be varied. To vary the mechanical pre-stress the clamping device can comprise piezoelectric or hydraulic elements, for example a hydraulic piston.
It is known that the frictional torque of a ball joint can be adjusted. Some systems for this make use of an adjustment drive which, to influence the frictional torque, exerts an axial force on the ball and its bearing. To determine the frictional torque a sensor system with an evaluation unit is required, which concludes what the frictional torque is on the basis of various measured parameters.
However, the frictional torque cannot be determined in every situation. Since it occurs only when the frictional elements move relative to one another, other parameters have to be found in order to determine or adjust it. These parameters can for example be the path to be covered by a pressure element, the counting of revolutions of a driving electric motor or the measurement of a motor current, which is brought into relationship with the frictional torque produced by means of an evaluation unit and a calculation. The primary aim of these options is to determine the axial force acting on the ball joint in order to be able to set the desired frictional torque. These derived parameters, however, are prone to large disturbing influences due to interfering forces from outside, wear in the ball joint, elastic effects and friction in the system, temperature dependencies and other influences which are difficult to allow for. Direct measurement of the force requires sensors which, although known from the field of measurement technology, are not accessible in the case of large production runs (such as in the automobile industry) for cost reasons.